With the advent of ever smaller geometries used in the manufacture of advanced VLSI components, the capacitance which occurs between adjacent metal connecting leads becomes greater. This increased capacitance leads to increased time delay for signals transmitted over these leads. It also contributes to greater power consumption by the chip and to cross talk between lines. Accordingly, designers have recognized that reducing the capacitance between closely adjacent conducting lines will have a significant effect on circuit performance. This can only be accomplished by changes in geometry of the physical layout of conductors in the chip or by decreasing the dielectric constant of the material which fills the space between adjacent lines. For this reason much research into low dielectric materials is underway in many laboratories around the world.
Measurement of the dielectric constant K for a material is usually performed by fabricating metal-insulator-semiconductor (MIS) capacitors. Once formed, the capacitors are contact probed at a testing station to measure K. This fabrication and probing is not a trivial process. For accurate determination of K, both the area of the metal electrode and the dielectric thickness underneath the electrode must be known. Use of shadow masks and direct lithography are the methods best suited to controlling the area. However, to assure high accuracy, the area of the metal electrode is also measured for each capacitor being measured. Optical ellipsometry or reflectometry is then used to determine film thickness. Accordingly, this technique is neither fast, non-destructive nor inexpensive.
Once the metal electrode for the capacitor is formed, the structure is permanent in that any etch which might be performed to remove the electrode may alter the dielectric's characteristics. The dielectric thus is disposed under a metal electrode and is not accessible for further evaluation or research. Therefore, measurement of the dielectric constant K with a MIS capacitor is considered to be a destructive technique.
Another common technique used for measuring the dielectric constant of a thin film is to use a mercury probe. In this method, a conductive column of mercury is put on the thin film and the capacitance is measured directly. This method is destructive inasmuch as mercury is considered a contaminant in most semiconductor processing facilities. Expensive handling and disposal laws and regulations contribute added cost for using this alternative dielectric measurement method.
Mercury probes, in addition, are inherently inaccurate due to a fundamental problem with the way mercury adheres to a surface. The surface which the mercury probe contacts on the thin film depends on the material characteristics of the interface between the thin film and the column of mercury. Different thin films will cause the surface area to have a different size, and, unless the difficult area measurements are made for the mercury probe, the data cannot be any more reliable than the error in the surface area. Thus mercury probes for dielectric constant measurement is expensive, destructive and inaccurate.
In view of the above mentioned and other problems associated with the known approaches for measuring dielectric constant, it is the principal objective of the present invention to provide a method for measuring dielectric constant which is suitable for use in connection with manufacturing semiconductor chips and the like.
It is a further objective of the invention to provide a non-contact method of measuring dielectric constant where there is no contact between instrument probes and the wafer.
It is still further an objective of this invention to provide a non-destructive method for measuring dielectric constant.